The present invention relates to thickened curable compositions, to methods of molding such thickened compositions, and to molded products prepared by these methods.
The use of polymeric materials as substitutes for such traditional structural materials as wood or metal has achieved widespread acceptance and continues to grow. Molded plastics, in particular, have become quite prevalent, especially in production of automobile body parts, household appliance casings, toys, and furniture. A large proportion of such molded plastics are produced from unsaturated polyester resin based molding compounds, two typically utilized forms being sheet molding compound (SMC) and bulk molding compound (BMC). Unsaturated polyester resin based molding compounds, however, tend to be tacky and exhibit non-uniform flow properties, therefore a thickening agent is generally added to such compounds to reduce stickiness as well as to assist in making the flow of the composition uniform at molding. Many techniques for thickening unsaturated polyester resin based molding compositions have been developed, although generally inorganic additives such as alkaline earth oxides and hydroxides, for example, magnesium and calcium oxides and hydroxides, are utilized. These techniques, however, are not without attendant difficulties, for instance, thickening may either be inadequate resulting in a molding composition which cannot be handled or thickening may continue beyond the desired level resulting in a very hard, brittle, unacceptable molding compound.
Although perhaps the best known and most commonly utilized, unsaturated polyester resin based molding compositions are not the only types of molding compositions which generally require thickening prior to molding. Moreover some of these types of molding compositions cannot be thickened conventionally by the addition of alkaline earth oxides and hydroxides because due to their monomeric content they lack the requisite functional groups. For example, Canadian Pat. No. 1,125,943 the disclosure of which is hereby incorporated by reference, is directed to such curable compositions which cannot be thickened in a conventional manner. Broadly stated, the curable composition of the aforesaid Canadian patent comprises a compound containing at least two polymerizable vinyl groups per molecule; a polydiene polymer; optionally a polymerizable monovinyl compound and alumina trihydrate.
There are known in the art other methods of thickening besides the use of alkaline earth oxides and hydroxides. For example, U.S. Pat. No. 4,289,684 to Kallaur teaches that unsaturated polyester resin based SMC can be thickened by the addition of a polyhydroxy polyacrylate or polyhydroxy polymethacrylate and an organic polyisocyanate. The reaction between the polyisocyanate and polyacrylate or polymethacrylate provides thickening. This patent however is very limited since it does not provide for reaction of other materials with the polyisocyanate. In connection with this use of polyisocyanate, Japanese Pat. Publication No. 22617/1970 teaches vulcanization of a rubber blend formed by blending and kneading a rubber material, a reinforcing filler and polybutadiene glycol with polyfunctional isocyanate. Vulcanization, however, differs greatly from cure of generally utilized molding compounds such as SMC, BMC, and the types of compositions disclosed in the aforesaid Canadian Pat. No. 1,125,943. The latter cure by free-radical initiated polymerization whereas vulcanization occurs by crosslinking of the unsaturated hydrocarbon chain of rubber with sulphur by application of heat. This patent is also very limited and extrapolation to free-radical initiated polymerizable molding compositions is not suggested.
In view of the limitations of these art-recognized techniques a method of thickening is needed which can be utilized with any free-radical initiated polymerizable composition.